1. Field of the Invention
The present invention relates to a light quantity control device adapted for use in a beam recording apparatus or the like.
2. Related Background Art
In the field of beam recording apparatus equipped with a light quantity control device there is already known a laser beam printer, in which a photosensitive member is scanned with a laser beam modulated according to input information to form an electrostatic latent image, and the latent image is rendered visible with magnetic developing powder, called toner, and is then transferred onto a recording material such as paper.
FIG. 12 illustrates an example of such a conventional laser beam printer, in which a photosensitive drum 1 having a surface semiconductor layer of selenium or cadmium sulfide is rotatably supported in a housing and rotated at a constant speed in a direction A. There are also provided a semiconductor laser 2 and a control circuit 2A for the intensity and on-off control of the laser beam from the laser 2.
The laser beam L emitted by the semiconductor laser 2 is expanded to a predetermined diameter by a beam expander 3, and then enters a polygon mirror 4 having plural mirror faces and rotated at a fixed speed by a scanner motor 5, whereby the laser beam is deflected by the polygon mirror to perform a scanning motion in the substantially horizontal direction. Then the beam is focused, by an imaging lens 6 of f-.theta. characteristic, as a light spot on the photosensitive drum 1 previously charged to a predetermined polarity by a charger 13.
The laser beam reflected by a mirror 8 is detected by a detector 7, whose detection signal determines the timing of modulation of the semiconductor laser 2 for obtaining desired light information on the photosensitive drum 1. On drum 1, an electrostatic latent image is formed by the scanning laser beam, according to the above-mentioned input information. The latent image is rendered visible by toner deposition in a developer unit 9, and the visible image thus obtained is transferred onto a recording material, usually paper, fed from a cassette 10 or 11. The recording material then passes a fixing unit 12 for fixing the image to the material, and is discharged to an unrepresented tray.
The semiconductor laser usually employed in such a laser beam printer has an I - l characteristic, or the relation of the laser driving current and the emitted quantity of light, as shown in FIG. 13. The semiconductor laser does not emit a beam until the current reaches a threshold value (I.sub.th), but emits the laser beam when the threshold value is exceeded. In such a beam emission state, the light quantity l shows a certain inclination .alpha. as a function of the laser driving current, and .alpha. is called slope efficiency.
In such a laser beam printer, the control circuit 2A controls the quantity of light emitted by the semiconductor laser 2 and determines the laser current I.sub.T so as to obtain a predetermined quantity of light l.sub.T, and this quantity of light is maintained constant by the supply of a constant laser current I.sub.T.
However the I - l characteristic of the semiconductor laser, which has an initial form A as shown in FIG. 14, during a constant current drive with a laser current I.sub.T for obtaining a predetermined light quantity l.sub.TA, may vary to a form B or C for example due to an increase in the chip temperature caused by the current in the semiconductor laser.
Through a normal operation can be achieved while the photosensitive drum is scanned with the necessary predetermined light quantity l.sub.TA, the above-explained latent image may not be formed if the I - l characteristic changes to B shown in FIG. 14 due to the increase in temperature so that the light quantity of the laser decreases to l.sub.TB despite the maintenance of the laser current at I.sub.T. On the other hand the laser chip may be destroyed if the quantity of light increases to l.sub.TC as indicated by C. The present applicant already proposed light quantity controlling devices in the U.S. Pat. Nos. 4,201,994 and 4,443,695 but a sufficient effect could not be achieved by the processes disclosed in these patent applications.